Quinoline-lactone derivatives as fluorescence sensors: ratiometric response and solvatochromic effects

A straightforward synthetic strategy for a series of structurally diverse quinoline-lactone fluorophores (5a-5e) is reported. The synthesized compounds were fully characterized, and their photophysical behavior was systematically evaluated in different solvents. The derivatives exhibited absorption maxima in the near-UV and visible regions (342.5-393.0 nm) and tunable fluorescence emission ranging from 393 to 528 nm, with fluorescence quantum yields ranging from 0.003 to 0.367 depending on the substituent and solvent environment. The photophysical properties showed a strong dependence on the electronic nature of the appended groups. Phenyl-triazole derivatives (5d and 5e) exhibited blue-shifted emission, whereas compounds bearing substituted indole or benzyloxy groups (5a-5c) showed pronounced red shifts and larger Stokes shifts. Notably, the phenyl-triazole derivatives displayed enhanced emission in protic media, particularly in water, which was attributed to hydrogen-bonding interactions and solvent-dependent stabilization of the emissive excited state. The acid-responsive properties of 5e were further investigated using trifluoroacetic acid (TFA). In DMSO, 5e exhibited a well-defined two-state ratiometric response with an isoemissive point and a midpoint apparent pKa index of 0.65, characterized by attenuation of the 405 nm emission band and the emergence of a lower-energy band at 536 nm. This halochromic transition showed reversibility upon sequential addition of triethylamine (TEA). In aqueous media, protonation disrupted the hydrogen-bonding network, resulting in fluorescence quenching accompanied by a bathochromic shift. These findings highlight the versatility of the quinoline-triazole framework and the potential of compound 5e as a platform for acidity monitoring and solvent-dependent fluorescence modulation.